Store `BigIntStorage.capacity` in header

[LiarPrincess]

From developer.apple.com/ManagedBufferPointer/Capacity:

This value may be nontrivial to compute; it is usually a good idea to store this information in the “header” area when an instance is created.

Under the hood

From ManagedBuffer.swift:

extension ManagedBufferPointer {
  @inlinable
  @available(OpenBSD, unavailable, message: "malloc_size is unavailable.")
  public var capacity: Int {
    return (
      _capacityInBytes &- ManagedBufferPointer._elementOffset
    ) / MemoryLayout<Element>.stride
  }
}

Where _capacityInBytes is defined as malloc_size:

static inline __swift_size_t _swift_stdlib_malloc_size(const void *ptr) {
  extern __swift_size_t malloc_size(const void *);
  return malloc_size(ptr);
}

Solutions

• Store additional member capacity: Int in Header. Header size: 2 words.
• Pack capacity inside existing header. Header size: 1 word. Layout [1bit - isNegative][31 bits - count][32 bits - capacity]

[LiarPrincess]

Additional reference: loftwareArrayOfOptional (Dave Abrahams thingy).